In extreme ultraviolet (EUV) lithography, an EUV reticle is typically used from which the EUV light is reflected in a pattern. To this end, the EUV reticle comprises a reflective mirror overlayed by a patterned absorbing layer. Today, TaBN is the most commonly used absorber for this purpose. However, the extinction coefficient of TaBN is such that an absorbing layer thickness in excess of 60 to 70 nm may be used to achieve a sufficient absorbance. At this thickness, 3D mask effects are common, such as different shadowing from different angles of incidence, and reduce the quality of the reflected pattern. 3D mask effects are for example discussed in Philipsen et al. (2017) (Philipsen, Vicky, et al. “Reducing EUV mask 3D effects by alternative metal absorbers.” SPIE Advanced Lithography. International Society for Optics and Photonics, 2017).
Materials with a higher EUV extinction coefficient than TaBN are known, such as Ag, Ni, In, Co, Sb, and Te, but come with their own challenges. For example In has a melting temperature of 165° C. and there is therefore a risk of pattern deformation inside the EUV lithography scanner. Sb and Te have too high vapor pressures for long-term thermal stability in the scanner. Ag, Ni, and Co form crystalline layers and are difficult to pattern. Ni and Co are particularly difficult to etch, where issues may include a lack of volatile compounds (making a chemical dry etch difficult), lack of etch selectivity with respect to other materials in the reticle (e.g. Ru), re-deposition of sputter residue, profile control being difficult to achieve, process complexity, etc. U.S. Pat. No. 9,612,522B2 discloses the use of some of these materials in an EUV mask blank.